Deadline Assignment in EDF Schedulers for Real-Time Distributed Systems

Real-time distributed systems contain end-to-end flows, which are distributed actions composed of sequences of tasks activated through messages. Such flows usually have an end-to-end deadline but the internal tasks and messages do not have specific timing requirements. However, if EDF schedulers are used, it is necessary to assign scheduling deadlines to tasks and messages, which is usually done by distributing the end-to-end deadline among them. Distributed systems may have synchronized global clocks or non-synchronized local clocks. This work studies the influence of the clocks, global or local, on the deadline-assignment algorithms. A study on the poor performance observed for EDF schedulers with local clocks is presented. Then, a significant optimization of the assignment algorithms is shown, in which an amount of end-to-end deadline larger than the established timing requirement is distributed among tasks and messages. With this technique, two new algorithms for deadline-assignment are proposed, showing that they outperform the existing ones by up to 23 percent of processor utilization in the case of local clocks. Finally, the influence of release jitter in this kind of EDF systems and the positive effects of eliminating it are also studied.